Method for forming layer of hemispherical grains and for fabricating a capacitor of a semiconductor device

ABSTRACT

A method for forming a layer of hemispherical silicon grains having a desired density and a desired shape in order to increase the surface area of the storage electrode of a capacitor. The method involves the formation of a thin oxide film over an under silicon layer to be formed with hemispherical silicon grains, so that the formation of those hemispherical silicon grains can be carried out in such a manner that the hemispherical silicon grains have a desired density and a desired shape under given conditions irrespective of whether or not the under silicon layer is doped with impurity ions and the crystalline structure of the under silicon layer.

This is a divisional of U.S. Application No. 08/871,842, filed Jun. 9, 1997, now U.S. Pat. No. 5,909,625.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming a layer of hemispherical grains and for fabricating a capacitor of a semiconductor device in the fabrication of a highly integrated semiconductor device, and more particularly to a method for forming a layer of hemispherical grains having a desired density and a desired shape in order to increase the surface area of the storage electrode of a capacitor.

2. Description of the Prior Art

The recent high integration trend of semiconductor devices inevitably involves a reduction in cell dimension. For this reason, it is necessary to form capacitors having a large capacitance while reducing topology. In the case of a dynamic random access memory (DRAM) device constituted by one transistor and one capacitor, in particular, it is important to increase the capacitance of the capacitor. In order to achieve an increase in capacitance, various methods have been proposed concerning the use of a material exhibiting a high dielectric constant, the use of a dielectric film having a small thickness and the use of a lower electrode having an increased surface area. Although various materials, such as Ta₂O₅, TiO₂ or SrTiO₃, have been proposed as the dielectric material exhibiting a high dielectric constant, their reliance and thin film characteristics have not been confirmed. The use of a dielectric film having a reduced thickness results in the easy damage of the dielectric film occurring during an operation of the semiconductor device using the dielectric film.

In order to increase the surface area of a storage electrode, various structures have also been proposed. For example, they include a stack structure, a fin structure and a cylinder structure.

For the stack and cylinder structures, a method has also been proposed, in which a layer of hemispherical grains are formed on the surface of a storage electrode in order to increase the surface area of the storage electrode.

In this case, the shape and density of hemispherical silicon grains may vary in accordance with the condition of a silicon layer which is used as an underlayer for the growth of those hemispherical silicon grains. For this reason, it is necessary to vary the condition for the formation of such hemispherical silicon grains in accordance with the concentration of impurity ions doped in the under silicon layer and the crystalline structure of the under silicon layer. However, this is troublesome. If no variation in the process condition for the formation of hemispherical silicon grains is made when the condition for the formation of the under silicon layer varies, it is then difficult By to obtain desired density and shape of hemispherical silicon grains.

SUMMARY OF THE INVENTION

An objective of the invention is to provide a method for forming a layer of hemispherical silicon grains, which involves the formation of a thin oxide film over an under silicon layer to be formed with hemispherical silicon grains, so that the formation of those hemispherical silicon grains can be carried out in such a manner that the hemispherical silicon grains have a desired density and a desired shape under given conditions irrespective of whether or not the under silicon layer is doped with impurity ions and irrespective of the crystalline structure of the under silicon layer.

In accordance with one aspect of the present invention, a method for forming a layer of hemispherical silicon grains over an under silicon layer is provided and comprises the steps of: removing a natural oxide film existing on the under silicon layer; forming a thin oxide film over the under silicon layer; and forming a layer of hemispherical silicon grains over the thin oxide film

In accordance with another aspect of the present invention, a method for fabricating a capacitor of a semiconductor device is provided and comprises the steps of: forming a planarizing insulating film over a semiconductor substrate, and forming a contact hole in the insulating film in accordance with an etch process using a storage electrode contact mask; depositing a polysilicon layer for a storage electrode over the resulting structure obtained after the formation of the contact hole, and patterning the polysilicon layer in accordance with an etch process using a storage electrode mask, thereby forming a polysilicon layer pattern; removing a natural oxide film existing on the polysilicon layer pattern; surface-processing the polysilicon layer pattern using a solution containing NH₄OH, H₂O₂ and H₂O in a desired ratio, thereby forming a thin oxide film having a uniform thickness over the polysilicon layer pattern; forming a layer of hemispherical silicon grains over the oxide film; removing unnesessary hemispherical silicon grains by using storage electrode mask; and sequentially forming a dielectric film and a polysilicon layer for a plate electrode over the hemispherical silicon grain layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objectives and aspects of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which:

FIGS. 1 and 2 are sectional views respectively illustrating a method for forming a layer of hemispherical silicon grains in accordance with an embodiment of the present invention; and

FIGS. 3 and 4 are sectional views respectively illustrating sequential steps of a method for fabricating a capacitor provided with a layer of hemispherical silicon grains in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate sequential steps of a method for forming a layer of hemispherical silicon grains in accordance with an embodiment of the present invention, respectively.

In accordance with this method, an underlayer 1 such as a polysilicon or amorphous silicon layer is first prepared. Removal of a natural oxide film (not shown) formed over the underlayer 1 is then carried out using a solution containing hydrofluorine (HF) and H₂O. A thin oxide film having a uniform thickness is then formed over the underlayer 1 using a mixed solution of NH₄OH and H₂O₂, as shown in FIG 1.

The removal of the natural oxide film may be achieved using HF vapor formed by spraying HF and H₂O in place of the solution containing HF and H₂O. A buffer oxide etchant containing HF and NH₄F may also be used.

The formation of the thin oxide film 2 may be carried out under the condition in which the mixed solution of NH₄OH and H₂O₂ is maintained at a temperature of 50° C. or less. A mixed solution of HNO₃ and H₂O₂ may be used in place of the mixed solution of NH₄OH and H₂O₂. The oxide film 2 has a very weak structure because it is formed over the polysilicon or amorphous silicon layer. Where the underlayer, which is a mono-crystalline silicon substrate, is processed using the mixed solution of NH₄OH and H₂O₂ at a temperature of 50° C. or less while the oxide film has a thickness of less than about 15 Å, it is possible to grow silicon grains over the underlayer 1 in such a manner that they are integral with the underlayer 1. In this case, an integral structure for a storage electrode is obtained.

Thereafter, a layer 3 of hemispherical silicon grains is formed to a thickness of 1,000 Å or less over the oxide film 2 at a deposition temperature of 550 to 585 ° C. in a furnace, as shown in FIG. 2. The loading of the wafer is carried out at a temperature of 400° C. or less under the condition in which N₂ gas has been supplied to the furnace.

The reason why the oxide film 2, which is formed using a chemical process, is used in place of the natural oxide film naturally grown over the underlayer 1 is that it has a uniform surface enabling the formation of hemispherical silicon grains 3 at a density larger and more uniform than that when using the natural oxide film.

Natural oxide film may be formed on only a portion of the underlayer in accordance with the surface condition of the underlayer. Such a natural oxide film may be thickened where its wafer is loaded in a furnace at a high temperature. In this case, the formation of hemispherical silicon grains may be carried out non-uniformly. Furthermore, the thick oxide film serves to electrically insulate the hemispherical silicon grains from the underlayer. As a result, it is improper to practically use the resulting storage electrode.

It is noted that where the hemispherical grain layer 3 is grown over the thin oxide film 2 formed over the underlayer 1 in accordance with the present invention, it can be electrically connected to the underlayer 1.

FIGS. 3 and 4 are sectional views showing the method of fabricating a capacitor of a semiconductor device respectively illustrating sequential steps of a method for forming a layer of hemispherical silicon grains over a thin oxide film formed over the surface of a storage electrode having a stack structure in accordance with another embodiment of the present invention.

In accordance with this method, a planarizing insulating film 12 is first formed over a semiconductor substrate 11, as shown in FIG. 3. The insulating film 12 is etched at a desired portion thereof in accordance with an etch process using a storage electrode contact mask, thereby forming a contact hole 13. A polysilicon layer for a storage electrode is then deposited over the resulting structure. The polysilicon layer is then patterned in accordance with an etch process using a storage electrode mask, thereby obtaining a polysilicon layer pattern 14. Thereafter, removal of a natural oxide film (not shown) formed on the polysilicon layer pattern 14 is carried out using a solution containing HF and H₂O.

The polysilicon layer pattern 14 is then surface-processed using a solution containing NH₄OH, H₂O₂ and H₂O in a desired ratio. As a result, a uniform oxide film 15 is formed over the surface of the polysilicon layer pattern 14.

Subsequently, a layer 16 of hemispherical silicon grains is formed with thickness of 1,000 Å or less over the oxide film 15 at a deposition temperature of 550 to 585° C. in a furnace, as shown in FIG. 4.

The loading of the wafer is carried out at a temperature of 400° C. or less under the condition that N₂ gas has been supplied in the furnace.

The resulting structure is then subjected to well-known subsequent processes required for the formation of a capacitor electrode. That is, the hemispherical silicon grain layer 16 is partially removed at its portion existing at an unnecessary region in accordance with an etch process using the storage electrode mask. A dielectric film is then formed over the resulting structure. Finally, a polysilicon layer for a plate electrode is deposited over the dielectric film.

As apparent from the above description, the present invention provides a method for forming a layer of hemispherical silicon grains, which involves the formation of a thin oxide film over an under silicon layer to be formed with hemispherical silicon grains, so that the formation of those hemispherical silicon grains can be carried out in such a manner that the hemispherical silicon grains have a desired density and a desired shape irrespective of whether or not the under silicon layer is doped with impurity ions and irrespective of the crystalline structure of the under silicon layer. Accordingly, an improvement in reproducibility is obtained.

Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

What is claimed is:
 1. A method for forming a layer of hemispherical silicon grains over an under silicon layer, comprising the steps of: removing a natural oxide film existing on the under silicon layer; forming a thin oxide film over the under silicon layer by using a mixed solution including HNO₃ and H₂O₂; and forming a layer of hemispherical silicon grains over the thin oxide film.
 2. The method in accordance with claim 1, wherein the removal of the natural oxide film is carried out using a solution of hydrofluorine and H₂O.
 3. The method in accordance with claim 1, wherein the removal of the natural oxide film is carried out using hydrofluorine vapor which is formed by spraying hydrofluorine and H₂O.
 4. The method in accordance with claim 1, wherein the removal of the natural oxide film is carried out using a buffer oxide etchant containing hydrofluorine and NH₄F.
 5. The method in accordance with claim 1, wherein the formation of the hemispherical silicon grain layer is carried out under the condition that a wafer including the underlayer to be formed with the hemispherical silicon grain layer is loaded in a furnace supplied with N₂ gas at a temperature of 400° C. or less.
 6. The method in accordance with claim 1, wherein the hemispherical silicon grain layer is formed to a thickness of 1,000 Å or less at a deposition temperature of 550 to 585° C.
 7. A method for forming a layer of hemispherical silicon grains over an under silicon layer, comprising the steps of: removing a natural oxide film existing on the under silicon layer using a buffer oxide etchant containing hydrofluorine and NH₄F; forming a thin oxide film over the under silicon layer by using a mixed solution including HNO₃ and H₂O₂; and forming a layer of hemispherical silicon grains over the thin oxide film.
 8. A method for forming a layer of hemispherical silicon grains over an under silicon layer, comprising the steps of: removing a natural oxide film existing on the under silicon layer; forming a thin oxide film over the under silicon layer by using a mixed solution including NH₄OH and H₂O₂; and forming a layer of hemispherical silicon grains over the thin oxide film.
 9. The method in accordance with claim 8, wherein the mixed solution of NH₄OH and H₂O₂ is maintained at a temperature of 50° C. or less. 